Ported engine constructions with low-tension compression seals

ABSTRACT

In ported engine constructions, cooling of piston crowns and cylinder liners results in reduction or elimination of bore/liner distortions, thus ensuring circularity of the bore/piston interface throughout engine operation. Consequently, the need for heavily-tensioned piston rings is eliminated. Such engine constructions incorporate annular low-tension compression seals on the pistons, which substantially reduce port bridge wear during all phases of engine operation while also limiting blow-by during combustion.

RELATED APPLICATIONS

The following patents and patent applications, all commonly assigned tothe assignee of this application, contain subject matter related to thesubject matter of this application:

U.S. Pat. No. 7,156,056, issued Jan. 2, 2007 for “Two Cycle, OpposedPiston Internal Combustion Engine”;

WO/2005/124124, published on Dec. 29, 2005 for “Improved Two Cycle,Opposed Piston Internal Combustion Engine”;

U.S. Pat. No. 7,270,108, issued Sep. 18, 2007 for “Opposed Piston,Homogeneous Charge Pilot Ignition Engine”;

WO/2006/105390 published on Oct. 5, 2006 for “Opposed Piston,Homogeneous Charge, Pilot Ignition Engine”;

U.S. Pat. No. 7,334,570, issued Feb. 26, 2008 for “Common Rail FuelInjection System With Accumulator Injectors”;

WO/2006/107892 published on Oct. 12, 2006 for “Common Rail FuelInjection System With Accumulator Injectors”;

U.S. Pat. No. 7,360,511, issued Apr. 22, 2008 for “Opposed PistonEngine”;

WO 2007/109122 published on Sep. 27, 2007 for “Opposed Piston Engine”;

U.S. Pat. No. 7,546,819 issued Jun. 16, 2009 for “Two Stroke,Opposed-Piston Internal Combustion Engine”;

U.S. Pat. No. 7,549,401 issued Jun. 23, 3009 for “Two-Cycle,Opposed-Piston Internal Combustion Engine”;

U.S. patent application Ser. No. 11/642,140, filed Dec. 20, 2006, for“Two Cycle, Opposed Piston Internal Combustion Engine”;

U.S. patent application Ser. No. 11/725,014, filed Mar. 16, 2007, for“Opposed Piston Internal Combustion Engine With Hypocycloidal Drive andGenerator Apparatus”, published as US/2007/0215093 on Sep. 20, 2007;

U.S. Pat. No. 7,591,235 issued Sep. 22, 2009 for “Opposed Piston EngineWith Piston Compliance”;

U.S. patent application Ser. No. 12/075,557, filed Mar. 12, 2008, for“Internal Combustion Engine With Provision for Lubricating Pistons”;

U.S. patent application Ser. No. 12/456,735, filed Jun. 22, 2009, for“Two-Cycle Opposed-Piston, Internal Combustion Engine”; and,

U.S. patent application Ser. No. 12/586,352, filed Sep. 21, 2009, for“Opposed-Piston Engine”.

BACKGROUND

The technical field relates to a ported internal combustion engine. Morespecifically the technical field relates to a ported internal combustionengine that incorporates low-tension compression seals to achieve highBMEP (brake mean effective pressure) operation. The technical field alsorelates to an opposed-piston, compression ignition engine in whichlow-tension compression seals are mounted to the opposed pistons so asto minimize port bridge wear during all phases of engine operation whilealso limiting blow-by during combustion.

A ported internal combustion engine is an internal combustion enginehaving a cylinder with one or more ports formed therein for the passageof air into and/or out of the bore. For example, the cylinder of atraditional opposed-piston engine includes exhaust and inlet ports castor machined into the cylinder near respective exhaust and inlet ends ofthe cylinder liner. Pistons disposed crown-to-crown in the liner's boretraverse the ports while moving through respective bottom dead center(BDC) positions. Rings are mounted to the pistons to maintain a sealbetween the pistons and the liner bore, which reduces the passage ofcombustion gasses between the pistons and the bore (blow-by). The ringsare heavily tensioned against the bore to accommodate bore/linerdistortion caused by thermal and mechanical stresses. Each piston andits rings traverse a respective port twice during every complete enginecycle. The heavy tension forces the outer surfaces of the rings into ahigh frictional engagement with the bore and with the port bridges,especially where the rings contact the edges of the port openings. As aconsequence, repeated transits by the rings over the ports result inexcessive and uneven port bridge wear, and, ultimately, early ringfailures. The exhaust piston rings suffer particularly heavy damage dueto the high temperatures encountered at the exhaust port.

As a result of low durability due to bridge wear, traditional portedengines have had very limited acceptance in the markets for land, air,and marine engines. Measures have been proposed to reduce the complexfrictional interface between the piston rings and the port bridges. Onesuch step includes excessive lubrication of the piston/bore interface.However, oil consumption in these cases is typically about 2% of fuelconsumption, as compared to portless engines in which oil consumption istypically about 0.1% of fuel consumption. Such high oil consumption isnot acceptable under modern emission standards. Other measures includerounding and/or ramping the outer edges of the rings, beveling the edgesof the port openings, and customizing the shapes of the port openings.However, these solutions add to manufacturing costs and will continue tohave only limited effectiveness so long as the rings are heavilytensioned.

Ported engine constructions have been proposed which incorporate pistonswith axially symmetrical construction and coolant structures for coolingpistons and cylinder liners that reduce or eliminate bore/linerdistortions throughout engine operation. Because these cooling designsmaintain circularity of the bore/piston interface longitudinally of thecylinder throughout engine operation, they eliminate the need forheavily-tensioned rings. An example of such a design in an opposedpiston engine construction is found in commonly-owned U.S. Pat. No.7,360,511, issued Apr. 22, 2008. Accordingly, we have realized thatported internal combustion engines in which circularity of thebore/piston interface is maintained during all phases of engineoperation are well-suited for low-tension piston compression seals whichsubstantially reduce wear on port bridges while also limiting blow-by.

SUMMARY

Low-tension compression seals provide an effective seal between thecylinder bore and piston thereby maintaining compression and preventingblow-by, while reducing or eliminating the problem of frictionalinteraction with the port bridges.

These objects are achieved by a compression seal device in a portedinternal combustion engine in which circularity is maintained betweenthe bore of a ported cylinder and an axially symmetrical piston having acompression seal mounted in an annular groove. The compression seal hasan annular bearing surface to maintain a sealing annular contact withthe bore, with no clearance between the bearing surface and the bore, inresponse to a residual low level of compression seal tension in thedirection of the bore when the piston is near a bottom dead center (BDC)position. The bearing surface maintains a sealing annular contact withthe bore, with no clearance between the bearing surface and the bore, inresponse to a high level of compression seal tension in the direction ofthe bore resulting from pressure of combustion acting against an innerperipheral surface of the compression seal when the piston is near a topdead center (TDC) position.

In a ported engine, a low-tension, essentially circular compression sealis compressed slightly when mounted to a piston so as to permit thepiston to be received in the bore of a ported cylinder where thecompression seal is very lightly loaded against the bore at an initiallow level of tension such that there is no clearance between the bearingsurface of the compression seal and the bore. During combustion, whenthe piston has moved through a top dead center (TDC) position thehigh-pressure compression gasses act upon the inner peripheral surfaceof the compression seal, which presses the bearing surface of the sealmore tightly against the cylinder bore at a higher level of tension,thereby firmly sealing the space between the bore and the piston andpreventing blow-by. As the piston approaches a port and the compressionpressure approaches ambient pressure, the compression seal relaxes toits low tension mode, thereby substantially reducing friction betweenitself and the port bridges as the port is traversed.

A manufacturing process yields annular compression seals which exhibitvery low tension when mounted to a piston in a ported cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a low-tension compression seal in anuncompressed condition; FIG. 1B is a plan view of the low-tensioncompression seal in a compressed condition; FIG. 1C is a side elevationview of the low-tension compression seal in the uncompressed state; andFIG. 1D is a side elevation view of the low-tension compression seal inthe compressed state.

FIG. 2 is a side sectional view of an inlet side of a ported cylindershowing a bore with a piston disposed therein; FIG. 2A is an enlargedview of a portion of the piston showing details of a compression sealmounted thereto.

FIG. 3 is a partial schematic illustration of opposed pistons near topdead center when combustion occurs.

FIG. 4A is a perspective view of a piston crown having a circumferentialgroove for seating a low-tension compression seal. FIGS. 4B, 4C, and 4Dare side sectional views of the piston crown of FIG. 4A showing threearrangements for mounting low-tension compression seals. FIG. 4E is sperspective view of a low-tension compression seal with a “Z-shaped”gap.

FIG. 5 is a schematic view of an optional piston squish zone to enhanceoperation of the low-tension compression seal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A low tension compression seal for use on a piston in a ported internalcombustion engine is illustrated in one or more of the above-describeddrawings, and is disclosed in detail in the following description.Preferably, a “low tension compression seal” is an annular device which,when mounted to a piston received in the bore of a ported engine, isloaded against the bore by a tension of 3 Newtons, or less, such thatthere is no clearance between the bearing surface of the compressionseal and the bore when the engine is not operating; more desirablystill, the low tension compression seal is loaded against the bore by atension of nominally 0 Newtons such that there is no clearance betweenthe bearing surface of the compression seal and the bore when the engineis not operating.

A low-tension compression seal (hereinafter, “compression seal”) isshown in FIG. 1A, where the view is toward a side surface of thecompression seal and FIG. 1C, where the view is toward an annular outerperipheral surface (hereinafter, a “bearing surface”) of the compressionseal. In these figures, the compression seal 100, which has a body 110with a generally circular or annular shape, is shown in a “free” statewhere it is has not been compressed diametrically and is not subject tocombustion pressures. A gap 113 opens between a bearing surface 114 andan annular inner peripheral surface 115. An alignment notch 116 is cutinto the inner peripheral surface 115. Preferably, the alignment notch116 is positioned at the inside end of the gap 113 in order to align thegap with a bridge of an inlet or exhaust port of a ported internalcombustion engine in which the compression seal is to be mounted.

As per FIGS. 1B and 1D, when the compression seal 100 is diametricallycompressed as it would be when mounted to a piston received in the boreof a cylinder, the bearing and inner peripheral surfaces defineconcentric circular shapes. An outside diameter 117 (D_(o)) is measuredwith respect to an edge of the bearing surface 114. An inside diameter118 (D_(i)) is measured with respect to an edge of the inner peripheralsurface 115. As seen in the elevation views of FIGS. 10 and 1D, thecompression seal body 110 has opposing side surfaces 119 and 120.Preferably, the side surfaces 119 and 120 are annular, flat and parallelto a horizontal surface, and essentially perpendicular to the bearingand inner peripheral surfaces 114 and 115. As per FIGS. 1B and 1D, thegap 113 in the compression seal 100 remains slightly open when thebearing surface 114 is seated against the bore; for example, the gap 113may be open by about 0.33 mm. This slight opening enables thecompression seal to undergo thermal expansion during engine operation.The annular compression seal 100 is preferably constituted ofconventional materials, such as steel or cast iron with an admixture ofcarbon and one or more alloys, and may be conventionally coated orplated, as required by any particular application.

FIG. 2 illustrates a piston and cylinder assembly of a ported internalcombustion engine in which a low tension compression seal 100 is mountedto a piston. Preferably, but not necessarily, the engine has anopposed-piston construction and operates with a two-stroke cycle.Further, in order to simplify the description, only one compression sealis shown, although this should not limit the application of theprinciples illustrated, as two or more compression seals can be mountedto the piston. FIG. 2 shows the inlet side of an opposed-piton cylinderassembly, with the understanding that the explanation applies to theexhaust side of the assembly in the same manner. In this regard, see,for example, FIG. 4A of US publication 2006/0157003 A1. In FIG. 2, theopposed piston engine includes a cylinder 210 with a piston 219 having acrown 220 and a skirt 230. The skirt 230 may be mounted to the crown 220in a compound piston structure, or formed as one piece with the crown,in a unitary piston structure. The cylinder 210 has an inlet port 222cast or machined into the cylinder near one end thereof. The inlet port222 is constituted of an annular sequence of openings 223 alternatingwith bridges 224 between the openings 223. The openings 223 permit thepassage of pressurized air from an inlet manifold 225 into the bore 226of the cylinder 210. The bore 226 is defined by the inside cylindricalsurface 227 of the cylinder 210. The bore 226 has a diameter 228. InFIG. 2, the crown 220 is near a bottom dead center (BDC) position.During engine operation, as the piston 219 reciprocates in the bore 226,the crown 220 moves between top dead center (TDC) and BDC positions,traveling back and forth across bridges 224 of the inlet port 222 duringeach cycle of operation. The front surfaces of the piston crowns may becontoured as seen in FIG. 2, or may be planar.

As per FIG. 2, each of the bore and piston has an axially symmetricalconstruction, and it is desirable that the axial symmetries of thecylinder bore 226 and the piston crown 220 be maintained substantiallythroughout all phases of the operational cycle of the engine. With thiscondition met, circularity of the piston/bore interface is ensured,thereby permitting the use of low-tension compression seals in a portedinternal combustion engine. Preferably, but not necessarily, respectivecooling structures are tailored to direct liquid coolant on the cylinderliner and the back surface of the piston crown in such a manner as toachieve this condition. For example, liquid coolant directed throughcylinder coolant channels 229 on the outer surface of the cylinder linerand a radially symmetrical piston cooling structure 231 on the backsurface of the piston crown, both taught in cross-referenced patentsU.S. Pat. Nos. 7,360,511 and 7,549,401 (incorporated herein by thisreference), maintain the axial symmetries of the bore 226 and the crown220. Together, these cooling structures direct liquid coolant so as tomaintain circularity of the cylinder bore/piston interface,longitudinally of the cylinder, during all phases of engine operation,thereby eliminating the need for high-tension piston rings.

Refer now to FIG. 2A, an enlarged view of the circled portion seen inFIG. 2, in which the spacing between the outer side surface of thepiston 219 and the bore of the cylinder 210 is exaggerated for a clearerunderstanding of the following description. As seen in FIG. 2A, acircumferential groove 235 is provided in the outer side surface of thepiston 219, preferably in or near the crown 220. For example, althoughnot necessarily, the groove may be constituted as a gap defined in acompound piston structure when the crown 220 is joined to the upper endof the skirt 230 at 236 (by threading, for example). Alternatively, thegroove may be machined or cast into a unitary piston structure. Ineither case, the compression seal 100 is mounted in the groove 235 inthe piston 219. The floor 237 of the groove 235 defines a groovediameter. The groove 235 has opposing walls 239 and 240, with the wall239 being relatively nearer to the crown 220 than the wall 240.

With reference to FIGS. 1A and 10, when the compression seal 100 is inits free state, the outside diameter of the compression seal is veryslightly larger than the diameter of the bore 226. Thus, as per FIGS. 1Band 1D, when the compression seal 100 is mounted in the groove 235 andcompressed slightly in order for the piston to be inserted into the bore226, the outside diameter of the compression seal 100 is equal to thediameter of the bore 226, in which case the annular bearing surface ofthe compression seal 100 is very lightly loaded against the bore so thatthere is unbreached low-tension contact between the bearing surface andthe bore. In other words, in the compressed state, there is a residuallow level of compression seal tension in the direction of the bore andthere is no clearance between the annular bearing surface and the bore.There is a slight opening in the bearing surface where the gap islocated; however, this opening becomes negligible when the compressionseal expands during engine operation in response to the heat ofcombustion. The inside diameter of the compression seal 100 is slightlygreater than the diameter of the groove 235 shown in FIG. 2A, and thethickness of the compression seal is slightly less than the width of thegroove.

Operation of a ported internal combustion engine with one or morelow-tension compression seals will now be described using an opposedpiston engine as an illustrative example. With reference to FIG. 3,before operation of the ported internal combustion engine commences,each compression seal 100 is loaded by a residual low level of tensionin the direction of the bore 226, which urges the bearing surface of thecompression seal into engagement against the bore, with no clearancebetween the bearing surface and the bore. When engine operationcommences, as the pistons 219 a and 219 b move toward their TDCpositions, air is compressed between the crowns 220. Combustion occursin the space 310 when fuel injected into the compressed air ignites asthe pistons move through their TDC positions. The pressure (F′) producedby combustion acts against the crowns 220, propelling the pistons towardtheir BDC positions. Some of the combustion gas pressure leaks behindeach compression seal 100 and acts against the inner peripheral surfaceof each compression seal to increase the level of tension in thedirection of the bore 226, which urges the bearing surface of thecompression seal into a highly-tensioned engagement against the bore,thereby effecting a tight seal that prevents blow-by. After combustion,as the pistons move away from each other and toward their BDC positions,the combustion pressure P continually drops until it reaches an ambientlevel. As the pressure P drops, the force acting against eachcompression seal falls until the bearing surface is once again loadedagainst the bore only by the residual low level of compression sealtension in the direction of the bore with no clearance between theannular bearing surface and the bore as the piston transits the portbridges.

FIGS. 4A-4D illustrate a crown 400 such as can be incorporated into theconstruction of each of a pair of opposed pistons. The pistonconstruction may include assembly of the crown to a skirt as taught incross-referenced patents U.S. Pat. Nos. 7,549,401 and 7,360,511, forexample. Alternately, the crown may be an inseparable element of apiston with a unitary construction. The crown preferably is constitutedof a durable material such as steel, cast iron, aluminum, or aluminumhybrid, as required by any particular application. For example, we haveused crowns made of 4130 alloy steel.

As per FIGS. 4B-4D, the crown 400 includes a front surface 402 whichfaces the combustion space and a back surface 404. As per FIG. 4A, theconstruction of the crown includes a coolant structure for radiallysymmetrical impingement cooling of the back surface 404. Such coolingmaintains axial symmetry of the crown 400 during all phases of engineoperation. In this regard, liquid coolant is conducted in a tubularpiston rod (as seen in FIG. 2, for example) to a central point 405 fromwhich streams of liquid coolant flow through a coolant structureconstituted of an radially-symmetric array of channels 406 centered onthe longitudinal axis of the crown to the interior surface of the skirt(not shown). The crown includes at least one circumferential groove 410for mounting a low-tension compression seal.

As per FIGS. 4B, 4C, and 4D, low-tension compression seals 100 aremounted to the crown 400 as a pair in a single circumferential groove410 in the side surface of the crown (FIG. 4B), as a pair mounted inseparate grooves 410 (FIG. 4C), or singly in a single groove 410 (FIG.4D). If mounted pair-wise, the low tension compression seals 100 of apair are mutually rotated so as to maintain their gaps 113 out ofalignment. Further, if necessitated by design or performancerequirements, one or more alignment protuberances (not seen) are formedin the floor of a groove 410 wherein a low tension compression seal 100is mounted. Each protuberance is shaped to fit into an alignment notch116 on the inside surface 115 (seen in FIG. 1A) of a compression seal. Agroove 410 formed to receive a pair of compression seals may have oneprotuberance, in which case, the gap and alignment notch of at least onecompression seal would be mutually offset. Any protuberance formed in agroove is located on the side of the crown so as to prevent acompression seal it retains from rotating in the groove and maintain thegap of the compression seal in alignment with a bridge of the port whichthe compression seal traverses. Such alignment prevents the gap frombeing damaged and/or worn by interacting with edges of port openings.

FIG. 4E illustrates a low-tension compression seal 450 with a “Z-shaped”gap 453 in which a top surface portion 455 of the gap overlays a bottomsurface portion 456 of the gap, thereby eliminating a path forcombustion gasses to escape through gap 453.

An optional opposed-piston configuration including low-tensioncompression seals is illustrated in FIG. 5. The crowns of a pair ofopposed pistons disposed in the bore of a cylinder liner are shown at ornear TDC, when fuel is injected between the crowns in order to initiatecombustion. Two low-tension compression seals 100 are mounted to eachcrown. As illustrated, the front surface of each crown has a concavecontour 510, preferably, although not necessarily, in the shape of asymmetrical hemispherical bowl. At the end of a compression stroke, theopposing concave contours 510 define a section of a spherical space 512into which a fresh charge of air has been compressed, and into whichfuel is injected to initiate combustion. The flat circular peripheries515 of the crowns are in close proximity at TDC and form an annularsquish zone around the quasi-spherical space. An opening 517 through thesquish zone is formed by opposing notches in the crown peripheries; theopening 517 is aligned with an injector nozzle 520 so that fuel can beinjected into the quasi-spherical space 512. When the front surfaces ofthe pistons are this closely spaced, little or no injected fuel is ableto pass through the squish zone onto the cylinder bore. One potentialbenefit realized thereby is reduction or elimination of unburned fuel onthe bore surface near TDC that could otherwise reduce the effectivenessof the low-tension compression seals at and immediately followingcombustion.

Manufacturing Application:

We have manufactured low-tension compression seals exhibiting anestimated residual low level of tension in the direction of the bore aslow as three (3) Newtons when compressed by a diametrically-appliedforce sufficient to reduce the gap in a seal as would occur in acylinder bore of 80 millimeters (mm); desirably, the gap is reduced toabout 0.3 mm. The starting material was a tube of 440A stainless steel83 mm in diameter. The tube was heated to 1800° F. and maintained atthat temperature for four (4) hours, oil cleansed, and then tempered at600° F. for four (4) hours, and again oil cleansed. The interior andexterior surfaces of the tube were then turn finished to an outerdiameter of 80.4 mm and an inner diameter of 73.9 mm. A low-tensioncompression seal was manufactured by cutting an annular piece with athickness of 1 mm from the finished tube with a numerically-controlledmill. The opposing side surfaces of the annular piece were lapped flatand the inside and bearing surfaces were deburred using a hand tool. PerISO 6621-4, 8.1, a half moon-shaped alignment notch was formed in theinner surface using a 2.38 mm end mill at 73.85 mm diameter. Thealignment notch was then located in the mill and the annular piece wassplit to form the gap. The split was made with a saw and the resultinggap was de-burred. (We formed Z-shaped gaps by use of an ElectricalDischarge Machine (EDM) with a “Z” shaped wire with which theoverlapping notch was cut). The annular piece was then mounted on amandrel and the bearing surface was lapped with an 80 mm diameter roundlapping tool until it was “light tight” per ISO 6621-4, 7.2. We notedthat oscillating the mandrel during lapping would impose a slight barrelshape on the bearing surface. Once light tightness was achieved, anitride layer was applied to the annular piece per ISO 6621-4, 10.3.2,NT070. Finally, a chromium nitride layer was deposited on the bearingsurface.

Referring now to FIGS. 1A and 2, when manufactured as disclosed, theoutside diameter of low-tension compression seals (D_(O)) for an 80 mmcylinder bore was typically 80.4 mm+0.05, −0.00 mm, and the insidediameter (D_(i)) of the cylinder bore was measured at 80.0 mm+/−0.0075mm. The typical thickness of the low-tension compression seals, measuredbetween the opposing side surfaces, was 1.0 mm. When such a low-tensioncompression seal was mounted to a piston disposed in the 80 mm diameterbore, the annular interface between the annular bearing surface and thebore was light tight, indicating that no clearance occurred between theannular bearing surface and the bore.

The scope of patent protection afforded the novel articles and methodsdescribed and illustrated herein may suitably comprise, consist of, orconsist essentially of the low-tension compression seal, piston, andported cylinder. Further, the novel articles and methods disclosed andillustrated herein may suitably be practiced in the absence of anyelement or step which is not specifically disclosed in thespecification, illustrated in the drawings, and/or exemplified in theembodiments of this specification. Moreover, although one or moreinventions are described with reference to preferred embodiments, itshould be understood that various modifications can be made withoutdeparting from the spirit of the invention. Any invention describedherein is limited only by the following claims.

The invention claimed is:
 1. A compression seal device in a portedinternal combustion engine in which circularity is maintained betweenthe bone of a parted cylinder and an axially symmetrical piston having acompression seal mounted in an annular groove, the compression sealhaving an annular bearing surface to maintain a seating annular contactwith the bore, with no clearance between the bearing surface and thebore, in response to a residual low level of compression seal tension inthe direction of the bore when the piston is near a bottom dead center(BDC) position, and to maintain a sealing annular contact with the bore,with no clearance between the bearing surface and the bore, in responseto a high level of compression seal tension in the direction of the boreresulting from pressure of combustion acting against an inner peripheralsurface of the compression seal when the piston is near a top deadcenter (TDC) position: wherein the low level of compression seal tensionis no more than 3 Newtons.
 2. The compression seal device of dam 1, inwhich the bearing surface has a barrel-shaped contour.
 3. Thecompression seal device of claim 1, further comprising a named renterbody which comprises steel or cast iron.
 4. The compression seal deviceof dean 1, in which tire crown of each piston composes steel and eachcompression seal composes steel.
 5. In an opposed-piston internalcombustion engine in which a pair of opposed pistons are deposed in thebore of a cylinder, tire improvement comprising: a coolant structure ineach piston for directing liquid coolant on a back surface of tire crownof the piston; a circumferential groove in each piston; eachcircumferential groove having a floor defining a groove diameter; and,each circumferential groove having disposed therein a pair of annularlow-tension compression seals, each annular low tension compression sealincluding an annular body with bearing and inner peripheral surfaces anda gap therebetween, the inner peripheral surface defining an insidediameter of the annular body, and a nitride coating on the annular body,wherein the inside diameter is greater than the groove diameter, eachbearing surface contacting the bore with no clearance between thebearing surface and the bore to response to a low level of tension inthe direction of the bore when the piston is near a bottom dead center(BDC) position, and contacting the bore with no clearance between thebearing surface and the bore, in response to a high level of tension inthe direction of the bore resulting from pressure of combustion actingagainst the inner peripheral surface when the piston is near a top deadcenter (TDC) position; and wherein for each annular compression seal thelow level of tension is nominally 0 Newtons.
 6. The improvement of claim5, further comprising: a front surface of each crown having a concavecontour substantially surrounded by a flat circular periphery such flatwhen the pistons are near top deed center positions, the opposingconcave contours define a space for compressing a change of air andreceiving fuel ejected to initiate combustion and the flat circularperipheries form an annular squish zone around the space; and an openingthrough the squish zone formed by opposing notches in the circularperipheries through which fuel can be injected into the space.
 7. In aninternal combustion engine to which at least one piston is disposed inthe bore of a cylinder having at least one port, the piston including atleast one circumferential groove having a floor and opposing walls, thepiston including a coolant structure for maintaining circularity of thebore/piston interface during operation of the engine, the improvementcomprising: at least one annular low tension compression seal disposedin the circumferential groove, the annular low tension compression sealincluding an annular body with bearing and inner peripheral surfaces anda gap therebetween, the inner peripheral surface facing toward the floorand defining an inside diameter of the annular body, a first annularside surface facing toward the first wall, and a second annular sidesurface facing toward the second wall, wherein the inside diameter isspaced from the floor of the groove when the bearing surface is tocontact with the bore; the bearing surface contacting the bore with noclearance between the bearing surface and the bore in response to a towlevel of tension in the direction of the bore when the piston is near abottom dead center (BDC) position, and contacting the bore with noclearance between the bearing surface and the bore, to response to ahigh level of tension to the direction of the bore resulting frompressure of combustion acting against the toner peripheral surface whenthe piston is near a top dead center (TDC) position; wherein thecompression seal includes a gap aligned with a respective bridge of theat least one port.
 8. The improvement of claim 7, wherein the low levelof tension tone mare than 3 Newtons.
 9. The improvement of claim 7, inwhich the bearing surface has a barrel-shaped contour.
 10. Theimprovement of claim 7, in which the gapped annular body comprises steelor cast iron.
 11. The improvement of claim 7, in which each pistoncomprises steel and each compression seal comprises steel.
 12. A methodfor operating an internal combustion engine to which at least one pistonis disposed in the bore of a cylinder with a port opening through thebore, the piston including at least one circumferential groove with afloor, the method comprising: cooing the cylinder and the piston tomaintain circularity of the interface between the bore and piston;contacting the bore with an annular bearing surface of a compress ionseal mounted to the circumferential groove, the compression seal havingan annular inner peripheral surface spaced from the floor; reciprocatingthe piston between top dead center (TDC) and bottom dead center (BDC)position to response to combustion in the bore; the bearing surfacecontacting the bore with no clearance between the bearing surface andthe bore in response to a low level of compression seal tension in thedirection of the bore when the piston traverses the port; the bearingsurface contacting the bore with no clearance between the bearingsurface and the bore, in response to a high level of compression sealtension in the direction of the bore resulting from combustion pressureacting against the inner peripheral surface when the piston moves awayfrom the TDC position; and, maintaining alignment between a gap in thecompression seal and a respective port bridge.
 13. The method of claim12, wherein the low level of tension is about 3 Newtons.
 14. A methodfor operating an opposed piston engine in which a pair of pistons isdisposed in opposition in the bore of a cylinder, the cylinder inductinginlet and exhaust ports operating through the bore, each pistonincluding a crown with a front face, a circumferential groove having afloor, and an annular compression seal mounted in the groove, eachcompression seal inducing an annular bearing surface and an annularinner peripheral surface space from the floor of the groove in which thecompression seal is mounted, the method comprising: cooing the cylinderand the crown of each piston to maintain circularity of the interfacebetween the bore and each piston, contacting the bore with the annularbearing surface of each compression seal; combusting a mixture of airand fuel in the bore, between front faces, as the pistons move throughrespective top dead center (TDC) positions; the annular bearing surfaceof each compression seal contacting the bore, with no clearance betweenthe annular bearing surface and the bore, in response to a high level ofcompression seal tension in the direction of the bore resulting fromcombustion pressure acting against the inner peripheral surface as thepiston moves away from a TDC position; reducing tire compression sealtension resulting from combustion pressure of each compression seal asthe piston on which it is mounted moves toward a BDC position; theannular bearing surface of each compression seal contacting the bore,with no clearance between the annular bearing surface and the bore, inresponse to a residual low level of compression seal tension in thedirection of the bore as the piston traverses a port; forming a spacebetween front faces for compressing the air as the pistons move throughthe TDC positions; forming an annular squish zone around the space; andinjecting the fuel into the space through the opening through the squishzone; and, maintaining alignment of a gap in each compression seal witha respective port bridge as the piston on which the compression seal ismounted traverses a port.
 15. The method of claim 14, therein theresidual tow level of compression seal tension is about 3 Newtons. 16.In an internal combustion engine in which at least one piston isdisposed in the bore of a cylinder having at least one port, the pistoninducing at least one circumferential groove having a floor and opposingwalls, the piston including a coolant structure for maintainingcircularity of the bore/piston interface during operation of the engine,the improvement comprising: at least one annular low tension compressionseal deposed in the circumferential groove, the annular low tensioncompression seal inducing an annular body with bearing and innerperipheral surfaces and a gap therebetween, the inner peripheral surfacefacing toward the floor and defining an inside diameter of the annularbody, a first annular side surface facing toward the first wall, and asecond annular side surface facing toward the second wall, wherein theinside diameter is spaced from the floor of the groove when the bearingsurface is to contact with the bore; the bearing surface contacting thebore with no clearance between the bearing surface and the bore toresponse to a low level of tension in the direction of the bore when thepiston is near a bottom dead center (BDC) position, and contacting thebore with no clearance between the bearing surface and the bore, toresponse to a high level of tension in the direction of the boreresulting from pressure of combustion acting against the innerperipheral surface when the piston is near a top dead center (TDC)position; wherein the low level of tension is no more than 3 Newtons.